1. Field of the Invention
The present invention relates generally to a method and apparatus for driving an image display panel, and more particularly to a method and apparatus for driving a display using capacitive light-emitting elements such as organic electroluminescence elements or the like.
2. Description of Related Art
An electroluminescence display comprised of a plurality of organic electroluminescence elements arranged in a matrix has drawn attention as a display which provides for low power consumption, high display quality, and reduced thickness. As illustrated in FIG. 1, such an organic electroluminescence element has a transparent substrate 100 such as a glass plate on which a transparent electrode 101 is formed; at least one organic functional layer 102 including an electron transport layer, a light emitting layer, a hole transport layer and so on laminated on the transparent electrode 101; and a metal electrode 103 laminated on the organic functional layer 102. The transparent electrode 101 serving as an anode is applied with a plus voltage, while the metal electrode 103 serving as a cathode is applied with a minus voltage, i.e., a direct current is applied across the transparent electrode and the metal electrode, to cause the organic functional layer 102 to emit light. By using an organic compound possibly expected to exhibit satisfactory light emitting characteristics for the organic functional layer, the electroluminescence display has become good enough to be fit for practical use.
The organic electroluminescence element (hereinafter simply called the xe2x80x9celementxe2x80x9d as well) may be electrically represented as an equivalent circuit as illustrated in FIG. 2. As can be seen from the figure, the element can be replaced with a circuit configuration composed of a capacitive component C and a component E of a diode characteristic coupled in parallel with the capacitive component. Thus, the organic electroluminescence element can be regarded as a capacitive light-emitting element. As the organic electroluminescence element is applied with a direct current light-emission driving voltage across the electrodes, a charge is accumulated in the capacitive element C. Subsequently, when the applied voltage exceeds a barrier voltage or a light emission threshold voltage inherent to the element, a current begins flowing from one electrode (on the anode side of the diode component E) to the organic functional layer which carries the light emitting layer so that light is emitted therefrom at an intensity proportional to this current.
The Voltage Vxe2x80x94Current Ixe2x80x94Luminance L characteristic of such an element is similar to the characteristic of a diode, as illustrated in FIG. 3. Specifically, the current I is extremely small at a light emission threshold Vth or lower, and abruptly increases as the voltage increases to the light emission threshold Vth or higher. The current is substantially proportional to the luminance. Such an element, when applied with a driving voltage exceeding the light emission threshold Vth, exhibits a light emission luminance in proportion to a current corresponding to the applied driving voltage. On the other hand, the light emission luminance remains equal to zero when the driving voltage applied to the element is at the light emission threshold Vth or lower which does not cause the driving current to flow into the light emitting layer.
As a method of driving a display panel using a plurality of organic electroluminescence elements as described above, a simple matrix driving mode may,be applied. FIG. 4 illustrates the structure of an exemplary simple matrix display panel. As can be seen, n cathode lines (metal electrodes) B1-Bn are arranged extending in parallel in the horizontal direction, and m anode lines (transparent electrodes) A1-Am are arranged extending in parallel in the vertical direction. At each of intersections of the cathode lines and the anode lines (a total of nxc3x97m locations), a light emitting layer of an organic electroluminescence element E1,1-Em,n is sandwiched between associated cathode line and anode line. The elements E1,1-Em,n carrying pixels are arranged in matrix, and each element has one end connected to an anode line (on the anode line side of the diode component E in the aforementioned equivalent circuit) and the other end connected to a cathode line (on the cathode line side of the diode component E in the aforementioned equivalent circuit) corresponding to the intersections of the anode lines A1-Am along the vertical direction and the cathode lines B1-Bn along the horizontal direction. The cathode lines are connected to a cathode line scanning circuit 1 and driven thereby, while the anode lines are connected to an anode line driving circuit 2 and driven thereby.
The cathode line scanning circuit 1 has scanning switches 51-5n corresponding to the cathode lines B1-Bn for individually determining potentials thereon. Each of the scanning switches 51-5n connects a corresponding cathode line either to a reverse bias voltage VCC (for example, ten volts) derived from a power supply voltage or to a ground potential (zero volt).
The anode drive circuit 2 has current sources 21-2m (for example, regulated current sources) corresponding to the anode lines A1-Am for individually supplying the elements with driving currents through respective anode lines, and drive switches 61-6n which are adapted to individually control on and off the currents flowing into the anode lines. While voltage sources such as regulated voltage sources could be used for the drive sources, current sources (power supply circuit controlled to supply a desired amount of current) are generally used for several reasons including the fact that the aforementioned current-luminance characteristic remains stable against temperature changes, whereas the voltage-luminance characteristic is unstable against temperature changes. The current sources 21-2m supply the associated elements with such amounts of currents that are required to maintain the respective elements to emit light at desired instantaneous luminance (hereinafter this state is called the xe2x80x9csteady light emitting statexe2x80x9d). Also, When an element is in the steady light emitting state, the aforementioned capacitive element C is charged with a charge corresponding to the amount of supplied current, so that the voltage across both terminals of the element is at a regulated value Ve (hereinafter, this value is called the xe2x80x9clight emission regulating voltagexe2x80x9d) corresponding to the instantaneous luminance.
The anode lines are also connected to an anode line reset circuit 3. The anode line reset circuit 3 has shunt switches 71-7m, disposed one for each anode line. Anode lines are connected to the ground potential, when associated shunt switches are selected.
The cathode line scanning circuit 1, the anode line drive circuit 2 and the anode line reset circuit 3 are connected to a light emission control circuit 4.
The light emission control circuit 4 controls the cathode line scanning circuit 1, the anode line drive circuit 2 and the anode line reset circuit 3 in accordance to the image data supplied from an image data generating system, not shown, so as to display an image represented by image data. The light emission control circuit 4 generates a scanning line selection control signal for controlling the cathode line scanning circuit 1 to switch the scanning switch 51-5n such that any of the cathode lines corresponding to a horizontal scanning period of the image data is selected and set at the ground potential, and the remaining cathode lines are applied with the reverse bias voltage VCC. The reverse bias voltage VCC is applied by regulated voltage sources connected to cathode lines in order to prevent crosstalk light emission from occurring in elements connected to intersections of a driven anode line and cathode lines which are not selected for scanning. The reverse bias voltage VCC is typically set equal to the light emission regulating voltage Ve (VCC=Ve). As the scanning switches 51-5n are sequentially switched to the ground potential in each horizontal scanning period, a cathode line set at the ground potential functions as a scanning line which enables the elements connected thereto to emit light.
The anode line drive circuit 2 conducts a light emission control for the scanning lines as mentioned above. The light emission control circuit 4 generates a drive control signal (driving pulse) in accordance with pixel information indicated by image data to instruct which of elements connected to associated scanning lines are driven to emit light at which timing and for approximately how long, and supplies the drive control signal to the anode line drive circuit 2. The anode line drive circuit 2, responsive to this drive control signal, controls on and off some of the drive switches 61-6m to supply driving currents to associated elements through the anode lines A1-Am in accordance with the pixel information. In this way, the elements supplied with the driving currents are forced to emit light in accordance with the pixel information.
The anode line reset circuit 3 performs its reset operation in response to a reset control signal from the light emission control circuit 4. The anode line reset circuit 3 turns on any of the shunt switches 71-7m corresponding to anode lines to be reset, indicated by the reset control signal, and turns off the rest of the shunt switches 71-7m.
Japanese Patent Kokai No. 9-232074 commonly filed by the present applicant discloses a driving method for use in a simple matrix display panel for performing a reset operation to discharge an accumulated charge on each of elements arranged in lattice form immediately before scanning lines are switched (hereinafter called the xe2x80x9creset driving methodxe2x80x9d). This reset driving method permits elements to trigger light emission earlier when scanning lines are switched. This reset driving method for a simple matrix display panel will be explained below with reference to FIGS. 4 to 6.
The operation illustrated in FIGS. 4 to 6, described below, is taken as an example in which a cathode line B1 is scanned to have elements E1,1 and E2,1 emit light, and subsequently, a cathode line B2 is scanned to have elements E2,2 and E3,2 emit light. Also, for facilitating the understanding of the explanation, assume that a diode symbol represents an element which is emitting light, while a capacitor symbol represents an element which is not emitting light. Further, a reverse bias voltage VCC applied to cathode lines B1-Bn is set at ten volts which is identical to a light emission regulating voltage Ve of the elements.
Referring first to FIG. 4, only a scanning switch 51 is switched to the ground potential equal to zero volt to scan a cathode line B1. The remaining cathode lines B2-Bn are applied with the reverse bias voltage VCC through the scanning switches 52-5n. Simultaneously, anode lines A1 and A2 are connected to current sources 21 and 22 through drive switches 61 and 62, respectively. The remaining anode lines A3-Am are switched to the ground potential at zero volt through shunt switches 73-7m. Thus, in the state illustrated in FIG. 4, the elements E1,1 and E2,1 only are forwardly biased so that driving currents flow thereinto from the current sources 21 and 22 as indicated by arrows, causing only the elements E1,1 and E2,1 to emit light. In this state, the elements E3,2 and Em,n which are not emitting light, indicated by hatching, are charged with polarities as indicated in the drawing.
Immediately before proceeding from the steady light emitting state of FIG. 4 to a state in which the next elements E2,2 and E3,2 are driven to emit light, the following reset control is performed. Specifically, as illustrated in FIG. 5, all the drive switches 61-6m are opened, all the scanning switches 51-5n and all the shunt switches 71-7m are switched to the ground potential at zero volt, and all of the anode lines A1-Am and the cathode lines B1-Bn are once shunted to the ground potential at zero volt, thus fully resetting the entire display panel. As a result of this full reset operation, all of the anode lines and the cathode lines are at the same potential equal to zero volt, so that charges stored in the respective elements are discharged through routes as indicated by arrows in the drawing, whereby the charges stored in all the elements are eliminated in a flash.
After eliminating the charges stored in all the elements in this way, only the scanning switch 52 corresponding to the cathode line B2 is next switched to zero volt to scan the cathode line B2, as illustrated in FIG. 6. Simultaneously with this, the drive switches 62 and 63 are closed to connect the current sources 22 and 23 to anode lines corresponding thereto, and the shunt switches 71, 74-7m are turned on to apply the anode lines A1, A4-Am with zero volt.
As is understood, the light emission control according to the reset driving method involves repetitions of a scanning mode which is a period in which any of the cathode lines B1-Bn is made active, and a reset mode subsequent thereto. The scanning mode and the reset mode are performed every one horizontal scanning period (1H) of image data. Assuming that a transition is made directly from the state of FIG. 4 to the state of FIG. 6 without performing the reset control, a driving current supplied from the current source 23, for example, not only flows into the element E3,2 but also is consumed for canceling reversely directed charges stored in the elements E3,3-E3,n (shown in FIG. 4), thereby requiring an extra time to bring the element E3,2 into the steady light emitting state (the voltage across both terminals of the element E3,2 is increased to the light emission regulating voltage Ve).
However, the above-mentioned reset control, if performed, results in the potentials at the anode lines A2 and A3 increased to approximately VCC at the moment the scanning is switched to the cathode line B2, so that the elements E2,2 and E3,2, which must be driven to emit light next time, are applied with charging currents flowing thereinto not only from the current sources 22 and 23 but also from a plurality of routes from regulated voltage sources connected to the cathode lines B1, B3-Bn. These charging currents charge parasitic capacitances to allow the voltages applied to the elements to instantaneously reach the light emission regulating voltage Ve, thereby accomplishing instantaneous transition to the steady light emitting state. Subsequently, since the amounts of current supplied from the associated current sources are enough for the elements to maintain the steady light emitting state at the light emission regulating voltage Ve during the scanning period of the cathode line B2, currents supplied from the current sources 22 and 23 flow only into the elements E2,2 and E3,2 and are consumed only for the light emission. In other words, the light emitting state illustrated in FIG. 6 is maintained.
As described above, according to the conventional reset driving method, since all of the cathode lines and the anode lines are once connected to zero volt or the ground potential or the potential equal to the reverse bias voltage VCC and reset before the transition to the light emission control for the next scanning line, the charging up to the light emission regulation voltage Ve can be achieved faster at the time the light emission control is switched to the next scanning line, thereby allowing elements on the switched scanning line, which should emit light, to trigger the light emission earlier.
The voltage levels on the cathode lines and the anode lines in the operations illustrated in FIGS. 4 to 6 can be represented in timing chart form as shown in FIGS. 7A, 7B. In a first scanning period, elements located at the intersections of the cathode line B1 and the anode lines A1, A2 are applied with a voltage across both terminals thereof at an anode line voltage level VAA (equal to Ve in FIGS. 4 to 6) and emit light at a luminance corresponding to this level VAA. In a second scanning period, element located at the intersections of the cathode line B2 and the anode lines A2, A3 are applied with a voltage across both terminals thereof at the anode line voltage level VAA (equal to Ve in FIGS. 4 to 6) and emit light at a luminance corresponding to this level VAA.
It should be noted that when the luminance is adjusted in a light emitting display employing the conventional reset driving method as described above, a luminance adjusting method common to matrix displays is applied for this purpose. Specifically, there are two modes for adjusting the luminance: a pulse width modulation mode and a pulse level modulation mode. The pulse width modulation mode is performed such that, as shown in FIG. 7A, the level of a voltage across both ends of an element during light emission is maintained at a fixed value (i.e., the element is driven to emit light at a fixed instantaneous luminance by a fixed driving current), and a connecting time period of a drive source to an associated anode line is varied within a range of the scanning period to adjust the luminance of light emitted by each element. The pulse level modulation mode, in turn, is performed such that, as shown in FIG. 7B, an anode line is connected to an associated drive source for a fixed duration corresponded to the scanning period, and the level of a voltage across both terminals of an associated element is varied every scanning period by the drive source (a driving current level is varied) to control the luminance of light emitted by each element. The method shown in FIG. 7A employs a regulated current source capable of supplying a fixed current at all times as the drive source since the elements have a fixed instantaneous luminance. The method shown in FIG. 7B, on the other hand, employs a variable current source as the drive source such that the elements have an instantaneous luminance which is fixed within a scanning period and variable from one scanning period to another. The luminance is reproduced by these methods.
The simple matrix display panel which executes the reset driving method as described above, however, has the following problems when the elements are adjusted for the luminance. The pulse width modulation mode as shown in FIG. 7A relies only on the length of the driving time for weighting of gradation, thus leading to a limited adjustable range and difficulties in reproducing multi-level gradation over a wide range. The pulse level modulation mode as shown in FIG. 7B, on the other hand, experiences difficulties in accurately adjusting the voltage level across both terminals of the elements during light emission every scanning period and as a result, suffers from a degraded linearity of luminance gradation. This is in part due to the use of the current sources as drive sources for driving the anode lines (drive sources controlled to supply a predetermined amount of current), and in part due to the levels of the voltages across both terminals of the elements unconditionally becoming substantially equal to the reverse bias voltage VCC at the moment a transition to a scanning period has been made through a reset period. Therefore, if the luminance is adjusted by varying the level of the voltage across both terminals of each element every scanning period as is the case of FIG. 7B, the voltage across both terminal of the element fails to have an ideal state as illustrated, thus resulting in the inability of accurately reproducing a desired luminance level.
FIG. 8 shows an actual level of a voltage across both terminal of an element, observed when the luminance is adjusted by the method illustrated in FIG. 7B. Specifically, in the simple matrix display panel employing the conventional reset driving method illustrated in FIGS. 4 to 6, the luminance adjusting method shown in FIG. 7B is implemented. As mentioned above, the luminance level L of an element has a value corresponding to the level of a voltage across both terminals of the element. Referring specifically to FIG. 8, light emission is conducted at a standard luminance during a jth scanning period; at a maximum luminance during a (j+1)th scanning period; and at a minimum luminance during a (j+2)th scanning period. The level of a voltage across both terminals of the element corresponding to a desired instantaneous luminance is Ve0 during the jth scanning period; Vemax during the (j+1)th scanning period; and Vemin during the (j+2)th scanning period. It should be noted that the voltage level Ve0 across both terminal of the element, when it emits light at the standard luminance, is set equal to the reverse bias voltage VCC, and the current sources 21-2m of the anode line drive circuit 2 are variable current sources which vary their respective amounts of supplied currents every scanning period (current sources controlled to supply the amount of current adjustable to a desired value).
As shown in FIG. 8, at the moment a transition to the jth scanning period has been made through a reset period, the potential level on an anode line connected to an element driven to emit light becomes substantially equal to the reverse bias voltage VCC in a flash, so that the level of a voltage across both terminals of the element is increased to approximately Ve0 just from the moment of the transition to the jth scanning period, thus enabling the element to emit light at a desired instantaneous luminance. Subsequently, since the element is supplied with and consumes a fixed amount of current from the variable current source only sufficient to emit light at the standard luminance, the element maintains the light continuously emitted at a constant luminance and the voltage level across both terminals thereof at Ve0.
Next, at the moment a transition to the (j+1)th scanning period has been made through a reset period, the potential level of the anode line is increased only to VCC as is the case of the jth scanning period, so that the level of a voltage across both terminals of the element does not reach the desired value, i.e., Vemax, resulting in the instantaneous luminance of the element lower than a desired luminance value. Subsequently, a current supplied from the variable current source distributively flows into parasitic capacitances of a plurality of elements connected to the drive line, and charges the parasitic capacitances, so that,the potential on the drive line is increased, and together with this, the voltage across both terminals of the element driven to emit light is also increased toward Vemax. However, the amount of current supplied from the variable current source is fixed in correspondence to the instantaneous luminance of the light emitted by the element during the (j+1)th scanning period. Thus, if the fixed amount of current flows into the parasitic capacitances of all elements connected to the drive line, the potential on the drive line will increase slowly, causing the voltage across both terminals of the element driven to emit light to similarly increase slowly as shown in FIG. 8. Then, at the time the potential on the drive line reaches Vemax, the voltage across both terminals of the element becomes stable. As a result, during the (j+1)th scanning period, the luminance is insufficient with respect to the desired luminance by a portion corresponding to a hatched area X, thus failing to reproduce the desired luminance.
Next, at the moment a transition to the (j+2)th scanning period has been made through a reset period, the potential level on the anode line is increased to VCC as is the case of the jth scanning period, so that the level of a voltage across both terminals of the element becomes larger than the desired value, i.e., Vemin, resulting in the instantaneous luminance of the element higher than a desired luminance value. Subsequently, since the amount of current supplied from the variable current source is smaller than that during the jth scanning, currents from scanning lines not selected as well as the current supplied from the variable current source attempt to flow into the element driven to emit light. This causes elements on the scanning line, not selected, to be gradually charged with charges of the opposite direction by the reverse bias voltage source, so that the potential on the drive line slowly drops, and a voltage across both terminals of the element driven to emit light also drops slowly as shown. Eventually, when the potential on the drive line reaches Vemin, the voltage across both terminals of the element becomes stable. As a result, during the (j+2)th scanning period, the luminance is excessive with respect to the desired luminance by a portion corresponding to a hatched area Y, thus failing to reproduce the desired luminance.
The present invention has been made in view of the problems described above, and its object is to provide a capacitive light-emitting element display device which is capable of extending a luminance adjustable range, and accomplishing a luminance adjustment with a good linearity.
In a first aspect, the present invention provides a method of driving a capacitive light-emitting element display device having a plurality of capacitive light-emitting elements arranged at a plurality of intersections of drive lines and scanning lines and connected between the scanning lines and the drive lines. The method includes the steps of connecting the scanning lines to either first or second potential, wherein the first and second potential are different from each other; connecting the drive lines to either the lower potential of the first and second potentials or a drive source; and in synchronism with a scanning period in which selected one of the scanning lines is connected to the lower potential of the first and second potentials, connecting selected one of the drive lines to the drive source to force a capacitive light-emitting element associated therewith to emit light, and simultaneously connecting the scanning lines, not selected, to the lower potential of the first and second potentials, wherein the higher potential of the first and second potentials is made adjustable.
The driving method further includes the step of resetting all the capacitive light-emitting elements during a reset period between the scanning periods.
The higher potential of the first and second potentials is made adjustable from one field period to another, and is maintained at a fixed potential during each field period.
The drive source may be a regulated current source.
The higher potential of the first and second potentials is adjusted within a range higher than a potential derived by subtracting a light emission threshold voltage from a light emission regulating voltage of the element, and the lower potential of the first and second potentials may be a ground potential.
Alteratively, the drive source may be a variable current source.
The higher potential of the first and second potentials is adjusted to be substantially equal to a light emission regulating voltage of the light emitting element, and the lower potential of the first and second potentials may be a ground potential.
During the reset period, the drive lines and the scanning lines is set at the same potential.
During the scanning period, the remaining drive lines except for the selected drive line connected to the drive source is connected to the lower one of the first and second potentials.
Preferably, the capacitive light-emitting elements are organic electroluminescence elements.
In a second aspect, the present invention provides a capacitive light-emitting element display device comprising a plurality of capacitive light-emitting elements arranged at a plurality of intersections of drive lines and scanning lines and connected between the scanning lines and the drive lines; scanning switch means for connecting the scanning lines to either first or second potential, wherein the first and second potentials are different from each other; drive switch means for connecting the drive lines to the lower potential of the first and second potentials or a drive source; light emission control means for controlling the drive switch means and the scanning switch means, wherein the light emission control means is operative in synchronism with a scanning period in which the scanning switch means connects selected one of the scanning lines to the lower potential of the first and second potentials for controlling the drive switch means to selectively connect the drive lines to the drive source to force selected capacitive light-emitting elements to emit light, and simultaneously controlling the scanning switch means to connect the scanning lines, not selected, to the lower potential of the first and second potentials; and adjusting means for adjusting the higher potential of the first and second potentials.
The light emission control means defines a period between the scanning periods for resetting all the capacitive light-emitting elements are reset.
Also, the adjusting means adjusts the higher potential of the first and second potentials from one field period to another, and maintains the higher potential at a fixed potential during each field period.
The drive source may be a regulated current source.
The adjusting means adjusts the higher potential of the first and second potentials within a range higher than a potential derived by subtracting a light emission threshold voltage from a light emission regulating voltage of the element, and the lower potential of the first and second potentials is a ground potential.
Alternatively, the drive source may be a variable current source.
The adjusting means.adjusts the higher potential of the first and second potentials to be substantially equal to the light emission regulating voltage of the light emitting element, and the lower potential of the first and second potentials is a ground potential.
The light emission control means sets the drive lines and the scanning lines at the same potential during the reset period.
The light emission control means connects the remaining drive lines except for the selected drive line connected to the drive source to the lower one of the first and second potentials during the scanning period.
Preferably, the capacitive light-emitting elements are organic electroluminescence elements.